Radiation curable pressure-sensitive adhesive composition

ABSTRACT

A radiation curable pressure-sensitive adhesive composition comprising a liquid oligomer having one or more acrylic double bonds in its molecule, a chain transfer agent and an aliphatic polar monomer having a carboxyl group can give an adhesive tape excellent in rubber-like properties, high in cohesive force and good in adhesiveness at low temperatures.

BACKGROUND OF THE INVENTION

This invention relates to a radiation curable pressure-sensitiveadhesive composition having high cohesion at low temperatures andexcellent adhesiveness.

Heretofore, in order to exhibit desired adhesive properties such ascohesive force and peel strength in pressure-sensitive adhesive tapes,it has been necessary to use a polymer having a relatively highmolecular weight and viscosity (e.g., natural rubber, synthetic rubber,etc.). Therefore, in coating, the polymer should be applied to asubstrate in the form of a solution in an organic solvent. Further,since a large amount of the solvent used is allowed to evaporate, a longperiod of time is required for a drying step after the coating.Moreover, solvents used for the above mentioned purpose are volatile andflammable and many of them have harmful effects on human body, so thattheir use often causes a fire or various environmental pollutionproblems. Further, a solvent-recovering apparatus which is regarded asnecessary from the viewpoint of economics and preventing theabove-mentioned environmental pollution is generally expensive andrequires a wide space for its provision. Accordingly, so-calledsolventless pressure-sensitive adhesives have been noted in recent yearsfrom the viewpoints of saving-resources, saving-energy andpreventing-environmental pollution. In order to make solventlessadhesives, there have been used emulsion type, hot melt type and thelike adhesives. Particularly, a radiation curable pressure-sensitiveadhesive applying a liquid oligomer having one or more unsaturateddouble bonds in its molecule is now of interest. This is because theradiation curable pressure-sensitive adhesive does not contain, inprinciple, an organic solvent which causes defects of theabove-mentioned solvent-type pressure-sensitive adhesives, or even if itcontains the solvent, the amount of the solvent is very small. Further,there are the following advantages: (1) since radiation which is actinicenergy rays is used, the curing (polymerization) reaction is fast, (2)since the curing reaction proceeds only when irradiated with radiation,the pot life can be controlled freely, (3) no large dryer is necessaryfor the production, etc.

But even the radiation curable pressure-sensitive adhesive has thefollowing disadvantages. The polymerization (curing) reaction generallyproceeds by addition of radicals generated by irradiation to unsaturateddouble bonds, but in this case the reaction rate is so fast that excesscrosslinking easily takes place, which results in raising the glasstransition point. Therefore, the formed adhesive layer film becomesbrittle, and in an extreme case, cracks are formed due to largepolymerization shrinkage. When adhesive layer film properties aremeasured in such a state, the film has large breaking strength buthardly shows elongation and rubber-like properties. On the other hand,when the absolute amount of unsaturated double bonds in the molecule isreduced in order to suppress the excess crosslinking, the reactionproceeds locally, which results in failing to obtain sufficient cohesiveforce as a whole.

As mentioned above, it as difficult to suppress the curing reactionsuitably while maintaining adhesive properties as a pressure-sensitiveadhesive.

In order to solve such problems, some of the present inventors haveproposed a pressure-sensitive adhesive comprising a radiation curableoligomer, a chain transfer agent and a polar group-containing monomer inorder to carry out the curing reaction uniformly and to enhanceinterfacial force (Japanese Patent Appln No. 128224/82, Japanese PatentExamined Publication No. 25070/85). But even such a pressure-sensitiveadhesive has a problem of lowering peel strength in a cold district orin the wintertime.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a radiation curablepressure-sensitive adhesive composition having excellent rubber-likeproperties and high peel strength and excellent adhesive properties atlow temperatures.

This invention provides a radiation curable pressure-sensitive adhesivecomposition comprising.

100 parts by weight of a liquid oligomer having one or more acryloyldouble bonds in its molecule,

0.2 to 20 parts by weight of a chain transfer agent, and

0.5 to 30 parts by weight of an aliphatic polar monomer having acarboxyl group and represented by the formula: ##STR1## wherein m is aninteger of 4 to 10; and n is an integer of 1 to 20.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The aliphatic polar monomer of the formula: ##STR2## wherein m is aninteger of 4 to 10; and n is an integer of 1 to 20, can be synthesizedby reacting a cyclic saturated aliphatic lactone of the formula:##STR3## wherein m is an integer of 4 to 10, with acrylic acid by aconventional process. In the formula (I), n is usually an integer of 1,but n can be an integer of 2 to 20 when side products are produced bythe synthesis.

Examples of the aliphatic polar monomer of the formula (I) are5-acryloyloxypentanoic acid, 6-acryloyloxyhexanoic acid,11-acryloyloxyundecanoic acid, etc.

The aliphatic polar monomer of the formula (I) is used in an amount of0.5 to 30 parts by weight per 100 parts by weight of the liquidoligomer. When the amount is less than 0.5 part by weight, the adhesiveproperties at low temperaures are lowered, while when the amount is morethan 30 parts by weight, the curing properties are lowered. Preferableamount of the aliphatic polar monomer is 1 to 20 parts by weight, morepreferably 5 to 15 parts by weight, per 100 parts by weight of theliquid oligomer.

The liquid oligomer having one or more acryloyl unsaturated double bondsin its molecule includes oligomers obtained by copolymerizing an alkylacrylate or alkyl methacrylate as a main chain in a low degree ofpolymerization, oligomers having one or more unsaturated double bonds inone molecule in a main chain or side chain such as polyol acrylates,polyester acrylates, urethane acrylates, and epoxy acrylates.

The oligomer is liquid under normal conditions and requires no soleventor if used in a very small amount. The viscosity of the liquid oligomeris in the range of preferably 10² cps to 5×10⁵ cps at room temperature(25° C.). Particularly preferable results can be obtained when theoligomer is highly reactive and has one or more acrylic double bonds.

Such an oligomer can be produced by the following processes.

(1) A process which comprises copolymerizing an acrylic ester compound,a monomer having a carboxyl group and if necessary, other monomers in asuitable organic solvent by a conventional solution polymerizationmethod, and then reacting a part of the carboxyl groups of the resultingcopolymer with a monomer having an epoxy group in the presence of apolymerization inhibitor and a catalyst to introduce one or moreolefinic unsaturated bonds to side chains.

(2) A process which comprises copolymerizing an acrylic ester compound,a monomer having an epoxy group, and if necessary, other monomers asmentioned above, and then reacting a part of the epoxy groups of theresulting copolymer with a monomer having a carboxyl group in the samemanner as described above to introduce one or more olefinic unsaturatedbonds into side chains.

(3) A process which comprises copolymerizing an acrylic ester compound,a monomer having a hydroxyl group and if necessary, other monomers asmentioned above, and then reacting a part of the hydroxyl groups of theresulting copolymer with a 1:1 addition reaction product of adiisocyanate such as tolylene diisocyanate, or hexamethylenediisocyanate and a monomer having a hydroxyl group (a half urethane) inthe same manner as described above to introduce one or more olefinicunsaturated bonds into side chains.

(4) A process according to (1), (2) or (3), wherein a monomer having anamino group is substituted for a part of the above-mentioned monomerhaving a carboxyl group in (1), monomer having an epoxy group in (2) ormonomer having a hydroxyl group in (3).

Examples of the acrylic ester compound are methyl acrylate, methylmethacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butylmethacrylate, isobutyl acrylate, isobutyl methacrylate, octadecylacrylate, octadecyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexylmethacrylate, etc. In this invention, it is preferable to contain 60% byweight or more of the acrylic ester compound in the main chain of theoligomer.

Examples of the monomer having a carboxyl group are acrylic acid,methancrylic acid, etc.

Examples of the monomer having an epoxy group are glycidyl acrylate,glycidyl methacrylate, etc.

Examples of the monomer having a hydroxyl group are 2-hydroxyethylacrylate, 2-hydroxylethyl methacrylate, 2-hydroxypropyl acrylate,2-hydroxypropyl methacrylate, etc.

Examples of the monomer having an amino group are dimethylaminoethylacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl acrylate,diethylaminoethyl methacrylate, etc.

As the chain transfer agent, there can be used thiol compounds such asbutyl mercaptan, lauryl mercaptan, octyl thioglycolate, ethylene glycoldithioglycolate, 1,4-butanediol dithiopropionate, trimethylolpropanetris(thioglycolate), trimethylolpropane tris(β-thiopropionate),pentaerythritol tetrakis(β-thiopropionate), etc., disulfides such asdiphenyl disulfide, etc.; halides such as carbon tetrachloride, carbontetrabromide, chloroform, etc. These compounds can be used alone or as amixture thereof.

The chain transfer agent is added to suppress excess crosslinking at thetime of radiation curing. The amount of the chain transfer agent is 0.2to 20 parts by weight, preferably 2 to 15 parts by weight, per 100 partsby weight of the liquid oligomer. When the amount is less than 0.2 partby weight, the effect for suppressing crosslinking becomes insufficient,whereas when the amount is more than 20 parts by weight, the adhesiveproperties are badly influenced due to insufficient curing andinsufficient cohesive force.

The radiation curable pressure-sensitive adhesive composition mayfurther contain one or more monomers for dilution, and if necessary, oneor more viscosity imparting agentC, softening agents, antioxidants,fillers, pigments, and the like additives.

The radiation used in this invention is actinic energy rays including anionizing radiation such as α-rays, β-rays, γ-rays, neutron rays,acceleated electron beams or the like. The ionizing radiation can beused in a dose in the range from 0.5 to 50 Mrad, preferably about 1 to20 Mrad. If necessary, the adhesive composition may contain a curingaccelerator. In the case of irradiation with radiation, care should betaken particularly in an irradiation atmosphere. Since the radicalsproduced by irradiation are badly influenced by oxygen in the air, theair may be replaced by an inert gas such as nitrogen gas, or in somecase, the film to be treated may be covered with a film or the like toremove oxygen.

This invention is illustrated by way of the following Examples, in whichall parts and percents are by weight unless otherwise specified.

EXAMPLES 1 TO 3, COMPARATIVE EXAMPLES 1 AND 2

A solventless acrylic copolymer was synthesized by bulk polymerizationof 80 parts of butyl acrylate and 20 parts of glycidyl methacrylate inthe presence of a catalyst. The resulting liquid oligomer had aviscosity of 5×10⁴ cps (30° C.). Then, 10 parts of acrylic acid wasaddition reacted to synthesize an acrylic radiation curable liquidoligomer having unsaturated double bonds at side chains. To 100 parts ofthe oligomer, 10 parts of ethylene glycol dithioglycolate as a chaintransfer agent and 20 parts of 5-acryloyloxypentanoic acid (Example 1),6-acryloyloxyhexanoic acid (Example 2) or 11-acryloyloxyundecanoic acid(Example 3) were added to give each radiation curable pressure-sensitiveadhesive composition.

For comparison, an adhesive composition was prepared in the same manneras described in Example 1 except for not using ethylene glycoldithioglycolate (Comparative Example 1).

Further, an adhesive composition was prepared in the same manner asdescribed in Example 1 except for using acid phosphoxydiethylmethacrylate (HOSMER M, a trade name, manufactured by Yushi Seihin K.K.)in place of the cyclic aliphatic lactone modified monomer (ComparativeExample 2).

Using the above-mentioned adhesive compositions, films were formed andtheir properties were tested as follows.

A film was formed by coating each composition on a release paper treatedwith a silicone in a constant thickness (about 1 mm), and irradiatingelectron beams from a linear filament type electron beam irradiatingapparatus (Electron Curtain, a trade name, manufactured by EnergyScience Inc.) at a dose of 5 Mrad with an accelerated voltage of 160 kVand a beam current of 10 mA in a nitrogen gas atmosphere (oxygenconcentration: 500 ppm) to carry out polymerization and crosslinking.Elongation and tensile strength of each adhesive layer film weremeasured by using a Tensilon tensile tester.

The results were as shown in Table 1. The rate of tension was 200 mm/min(20° C.).

                  TABLE 1                                                         ______________________________________                                                                 Tension strength                                     Example No.  Elongation (%)                                                                            (kg/cm.sup.2)                                        ______________________________________                                        Example 1    800         5.70                                                 Example 2    820         5.55                                                 Example 3    850         5.50                                                 Comparative  450         5.85                                                 Example 1                                                                     Comparative  800         5.45                                                 Example 2                                                                     ______________________________________                                    

It was found that the compositions of each Example had rubber-likeproperties sufficiently. Therefore, adhesive tapes were produced bycoating each composition on a polyester film of 0.025 mm in thickness(Lumilar #25, a trade name, manufactured by Toray Industries, Inc.) soas to give an adhesive layer of 0.015 mm thick, and irradiating thecomposition with electron beams in the same manner as described above.Peel strength and cohesion of the adhesive tapes were measured. Theresults were shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                          Change of                                                                     peel strength                                                  Initial peel                                                                          Adhesion at                                                                          with the                                                       strength.sup.(1)                                                                      low temp..sup.(2)                                                                    lapse of                                                                             Cohesion test.sup.(4)                            Example No.                                                                          (g/25 mm wide)                                                                        (0° C./20° C.)                                                         time.sup.(3) (%)                                                                     (mm/10 × 20 mm)                            __________________________________________________________________________    Example 1                                                                            440     1.8    105    0.02                                             Example 2                                                                            450     2.0    105    0.02                                             Example 3                                                                            420     2.5    106    0.02                                             Comparative                                                                          300     0.3    110    0.03                                             Example 1                                                                     Comparative                                                                          420     0.5    108    0.02                                             Example 2                                                                     __________________________________________________________________________     Note to Table 2:                                                              .sup.(1) Measured according to JIS C2107 (20° C.) (Adhered: SUS 43     BA plate).                                                                    .sup.(2) An adhesive tape was adhered to the SUS plate at 0° C. an     peel strength was measured at 0° C. and compared with the value        obtained in above .sup.(1). (The larger the value, the better.)               .sup.(3) A heat accelerating test at 65° C. was carried out for 7      days to measure an increase of peel strength in percentage. (The smaller      the increase, the better.)                                                    .sup.(4) An adhesive tape was adhered to a phenol resin plate and a slide     distance after 30 minutes under a load of 500 g at 20° C. was          measured.                                                                

As is clear from Table 2, the pressure-sensitive adhesive composition ofthis invention can give adhesive tapes having constant cohesive force,good in adhesiveness at low temperatures and little in the change withthe lapse of time.

EXAMPLES 4 TO 6, COMPARATIVE EXAMPLES 3 AND 4

An acrylic radiation curable liquid oligomer was synthesized bypolymerizing 40 parts of butyl acrylate, 40 parts of 2-ethylhexylacrylate and 20 parts of glycidyl methacrylate in the same manner asdescribed in Example 1 and adding 10 parts of acrylic acid thereto.

Each radiation curable pressure-sensitive adhesive composition wasprepared by adding 10 parts of carbon tetrabromide and 10 parts of5-acryloyloxypentanoic acid (Example 4), 6-acryloyloxyhexanoic acid(Example 5) or 11-acryloyloxyundecanoic acid (Example 6) to theoligomer.

For comparison, an adhesive composition was prepared in the same manneras described in Example 4 except for not adding carbon tetrabromide(Comparative Example 3), or except for using acryloyloxyethyl hydrogenphthalate in place of the cyclic aliphatic lactone modified monomer(Comparative Example 4).

The properties of adhesive layer films obtained from individual adhesivecompositions were tested. Further, each adhesive composition was coatedon a polyethylene film of 0.06 mm thick so as to give an adhesive layerof 0.005 mm, followed by irradiation with electron beams to prepare anadhesive film for surface protection.

Properties of the resutling adhesive films were tested and shown inTable 3.

                                      TABLE 3                                     __________________________________________________________________________                                   Comparative                                                                          Comparative                             Example No.  Example 4                                                                           Example 5                                                                           Example 6                                                                           Example 3                                                                            Example 4                               __________________________________________________________________________    Elongation (%)                                                                             900   950   960   480    900                                     Tension strength (kg/cm.sup.2)                                                                7.20                                                                                7.26                                                                                7.3                                                                                 6.45                                                                                 7.3                                  Initial peel strength                                                                      120   110   105    80    115                                     (g/25 mm)                                                                     Adhesion at low temp.                                                                         2.2                                                                                 2.4                                                                                 2.6                                                                                 0.4    0.7                                  (0° C./20° C.)                                                  Changes of peel strength                                                                   105   105   108   115    105                                     with the lapse of time                                                        (%)                                                                           Erichsen squeezing test.sup.(5)                                                            No change                                                                           No change                                                                           No change                                                                           Wholly No change                                                              peeled off                                     __________________________________________________________________________     Note:                                                                         .sup.(5) A SUS plate to which each adhesive film had been attached was        subjected to a 8 mm Erichsen squeezing test according to JIS B7777, and       the conditions of spontaneous peeling off of the film after 24 hours was      observed.                                                                

As is clear from Table 3, the pressure-sensitive adhesive composition ofthis invention can give adhesive tapes good in adhesion at lowtemperatures, little in the change of adhesive force with the lapse oftime and good in Erichsen squeezing test.

What is claimed is:
 1. A radiation curable pressure-sensitive adhesivecomposition comprising:100 parts by weight of a liquid oligomer havingone or more acryloyl double bonds in its molecule, 0.2 to 20 parts byweight of a chain transfer agent, and 0.5 to 30 parts by weight of analiphatic polar monomer having a carboxyl group and selected from thegroup consisting of 5-acryloyloxypentanoic acid, 6-acryloyloxyhexanoicacid and 11-acryloyloxyundecanoic acid.
 2. A composition according toclaim 1, wherein the liquid oligomer has a viscosity of 10² to 5×10⁵centipoises at room temperature.
 3. A composition according to claim 1,wherein the chain transfer agent is a thiol compound, a disulfide or ahalide.
 4. A composition according to claim 1, wherein the liquidoligomer is a copolymer obtained from monomers selected from the groupconsisting of butyl acrylate, glycidyl methacrylate, 2-ethylhexylacrylate and acrylic acid.
 5. A composition according to claim 1,wherein the liquid oligomer is obtained by copolymerizing an alkylacrylate or alkyl methacrylate as a main chain in a low degree ofpolymerization.
 6. A composition according to claim 1, wherein theliquid oligomer is an oligomer having one or more unsaturated doublebonds in one molecule in a main chain or side chain.